Myocardial delayed enhancement images acquired with segmented ECG-gated inversion recovery (IR)-prepared sequences often exhibit bright ghosting artifacts which impede the identification of infarcted territory. The artifacts arise from body fluids with long T1 values (e.g. pericardial effusion, cerebrospinal fluid CSF, or pleural effusion). If the amplitude of the oscillating signal could be reduced or if the oscillation could be avoided altogether, the ghost would be virtually eliminated.
In more detail, magnetic resonance imaging (MRI) is a safe, noninvasive test that creates detailed images of organs and tissues. “Noninvasive” means that no surgery is done and no instruments are inserted into the body. MRI uses radio waves and magnets to create images of organs and tissues. Unlike computed tomography scans (also called CT scans) or conventional x rays, MRI imaging doesn't use ionizing radiation or carry any risk of causing cancer.
As one example, cardiac MRI uses a computer to create images of the heart as it is beating, producing both still and moving pictures of the heart and major blood vessels. Doctors use cardiac MRI to get images of the beating heart and to look at the structure and function of the heart. These images can help doctors decide how best to treat patients with heart problems. For example, cardiac MRI is a common test for diagnosing and evaluating a number of diseases and conditions, including:
Coronary artery disease
Damage caused by a heart attack
Heart failure
Heart valve problems
Congenital heart defects
Pericardial disease (a disease that affects the tissues around the heart)
Cardiac tumors
Others.
In MRI, data is generally acquired by a software program called a pulse sequence running on an MRI scanner. Radio frequency and magnetic field “pulses” are used for transmitting energy and for spatial encoding, hence the name “pulse sequence.” The loud humming, tapping and buzzing sounds that emanate from the MRI equipment during testing are humanly-perceivable manifestations of the generation of these radio and magnetic field pulse sequences.
The scanner data is generally acquired in a raw format and placed in raw data-space called k-space. The raw data in k-space is then subjected to a mathematical operation called image reconstruction, which yields images showing the examined region of the human body. MRI systems can typically provide many subsets of pulse sequences (data acquisition software) that are tailored to certain parts of the body (e.g., the heart) and the examined pathophysiologies.
Natural motion and flow in the human body such as cardiac contraction and aortic blood flow generally occur too fast to be accurately captured within a single shot. Images with artifacts showing blurred moving structures similar to photos with camera shake would result. Therefore, many MRI techniques divide the data acquisition into repeated acquisition of small data portions of the entire data set, because less data can be acquired in less time resulting in less “camera shake”. The acquisition period to acquire a small data portion can be e.g. a heartbeat. These small data portions are called segments and the acquisition is known as segmented acquisition. Such segmented readout sequences are abundantly used throughout the entire body and in particular for cardiac and neuro MRI. For example, in cardiac MRI, in each heartbeat it may be that only 10% of the entire raw data is found. To find the missing data, i.e., the other 90%, the acquisition is repeated in the next nine heartbeats recording a different segment during each of a number of heartbeats until all raw data is found.
To make sure the heart or other organ being imaged is in the same position each time a segment is recorded, the acquisition is typically synchronized or “gated” to a signal obtained from the body. Such gating is a commonly used technique in MRI, and typically involves placing electrodes on the patient's skin. These electrodes pick up the faint electrical signals the patient's nervous system (electrical activity in the heart) generates. The MRI data acquisition can be triggered or gated by these faint electrical signals to synchronize the data acquisition with for example the beating of the heart. Thus for example, in the case of cardiac MRI, the acquisition can be synchronized with cardiac contraction by means of gating to an electrocardiogram (ECG). Gated segmented acquisitions are also used for various regions of the body such as the head, spine, and abdomen.
A schematic representation of an exemplary illustrative non-limiting “gated segmented” pulse sequence is seen in FIG. 1. The “gating” refers to triggering or synchronizing the pulse sequence and associated data capture (for example, to heart beat in response to electrocardiogram signals that stimulate the heart to beat). In the example shown, a first data segment (“segment 1”) is acquired (“DA” is “data acquisition”) during a particular time period in the first heartbeat R(1) to R(2), the second data segment is acquired during a particular time period in the third heartbeat R(3) to R(4), a third data segment is acquired during a particular time period in the fifth heartbeat R(5) to R(6), and so on.
Rather than gating to an ECG or another physiologic signal, one can also acquire the next segment after a fixed time delay and repeat this until all segments are acquired. The segments are thus recorded in a periodic pattern in a way that does not require gating. To image non-moving organs, segmented techniques can be employed as well. Instead of gating to a physiologic signal (e.g. a patient's ECG), fixed imaging periods are used. The resulting artifacts due to long T1-species are the same.
As discussed above and shown in FIG. 1, the segmenting in the cardiac MRI context refers to acquiring only part of the data during any given heartbeat, and repeating the data acquisition over multiple heartbeats (e.g., with data being captured every heart beat, every other heartbeat, or the like). Since the patient is generally asked to hold her breath during the segmented data acquisition, there is a practical limit to the number of heartbeats in the data collection. Generally speaking, fewer heartbeats are better because this reduces the length of time the patient needs to hold her breath.
One example of such an artifact creating sequence is the inversion recovery spoiled gradient echo sequence also known as IR Turbo FLASH (Fast Low Angle Shot) sequence with magnitude reconstruction. This pulse sequence (shown in FIG. 2) provides a non-selective inversion (IR) pulse before each data acquisition. In the cardiac MRI context, the IR TurboFLASH sequence uses an individual adaptation of the inversion time (TI) between the inversion pulse and data acquisition to achieve optimal signal increase between infarcted and viable myocardium. When a contrast agent is used, the infarcted tissue recovers from the inversion pulse at a different rate as compared to healthy tissue recovery. At the optimal TI, the signal intensity of normal myocardium is nulled. Several breath holds can be necessary to determine the optimal TI value as the acquisition of each image requires a breath hold.
The IR Turbo FLASH sequence has been highly successful in a variety of imaging contexts including cardiac imaging. While this and other segmented sequences are now widely used, there can be some problems in certain applications such as cardiac and other imaging, as will now be explained. In particular, image artifacts in MRI or signal artifacts in spectroscopy or NMR can be caused by such a gated and/or periodic segmented data acquisition when long-T1 species are implicated.
FIG. 3 shows the temporal evolution of an exemplary illustrative non-limiting longitudinal magnetization created by the IR Turbo FLASH sequence. In the example shown, the longitudinal magnetization does not follow the same recovery curve during each of the first five readouts (see arrows in the grey overlay region during the first 7.5 seconds of the sequence). The magnetization has a different polarity at each of the five data acquisition windows. In other words, the acquired magnetization of the long-T1 species is not at steady state, unlike that of the infarcted, and normal myocardium.
Not being at steady state can occur in different sequences and with as well as without magnetization preparation. For example, a so-called “steady-state-free-precession” sequence (SSFP) is actually not at steady state immediately after its start, unless an initial series of dummy pulses is played to drive it to steady state. Even then, matter with very long T1 may not be at steady state. The longer the longitudinal recovery time constant T1 is, the longer it takes the longitudinal magnetization to reach the steady state. These variations in magnetization during segment readout are not without consequences. Imaged matter (fluids, tissue, etc.) with long T1 values including pericardial effusion, pleural effusion, CSF around the brain and spine, stomach fluid, and saline in breast implants, are all prone to this effect.
The magnetization variations shown in FIG. 3 can produce extraneous image elements known as “image artifacts” or “ghost images” that may obscure the true image. Such artifacts have been described for example in a 2005 journal publication Kellman P, Dyke C K, Aletras A H, McVeigh E R, Arai A E, “Artifact Suppression in Delayed Hyperenhancement Imaging of Myocardial Infarction using BI-weighted Phased Array Combined Phase Sensitive Inversion Recovery,” Magn. Reson. Med. 2004 February; 51(2): 408-412 and elsewhere, and are commonly seen by radiologists.
One type of artifact stems from species (fluids, tissues) in the body with a long longitudinal recovery time (T1). For example, imaged species with a long T1 include fluids such as pericardial and pleural effusion, cerebro-spinal fluid (brain and spinal canal), and saline in breast implants. Effusions can also occur in other parts of the human body and would cause the same kind of artifact. The artifacts appear if long-T1 species are present in the imaged region and if the images are obtained with an inversion recovery pulse sequence using a so called segmented acquisition as described above that records the different parts of the raw data space (the segments) in a periodic fashion by repeatedly playing out the same scheme of inversion pulse and readout events, but acquiring a different set of lines during every readout. The artifact is referred to as ghosting and is due to the long-T1 species not being at steady state. The structure that contains the long-T1 species is superimposed as “ghosts” at multiple and erroneous locations throughout the image thereby obscuring the patient's morphology.
Image artifacts due to not being at steady state can occur in other sequences as well. For example, a so-called “steady-state-free precession” sequence (SSFP) is not actually at steady state immediately after its start, unless an initial series of dummy pulses is played to drive it to steady state. Even then, matter with very long T1 may not be at steady state. See FIG. 3. The longer the longitudinal recovery time constant T1 is, the longer it takes the longitudinal magnetization to reach steady state. Therefore, fluids with long T1 values called long-T1 species, including pericardial effusion, pleural effusion, CSF around the brain and spine, saline in breast implants, and stomach fluid, are prone to cause such artifacts even when dummy pulses are played.
As FIG. 3A further illustrates, the reason for the creation of long-T1 species ghosts is that these species are in a transient longitudinal magnetization state either during the beginning or throughout the entire duration of the scan. The magnetization transition can be oscillatory (as seen in FIG. 3A), decaying, increasing, or of any other nature. To the image reconstruction these transitions look like sinusoidal waves that modulate the original signal and create additional long-T1 species regions at erroneous ‘ghost’ locations that are related to the frequencies of the contained sinusoidal waves.
FIG. 4 shows two example images acquired by two exemplary pulses sequences; by a gated segmented inversion recovery (IR) (a) gradient echo and (b) steady state free precession (SSFP) sequence. The patient images contain multiple ghosting artifacts (circled) of fluid in the spinal canal. The artifacts occur if the longitudinal magnetization during data readout varies from one data acquisition (DA) to the next. The magnetization is in a transient rather than a steady state. “Steady” does not mean that the magnetization value is constant over time. Rather it means that the magnetization cycles through the same recovery curve during each imaging period and thus has the same value during each of the periodic data acquisitions.
FIGS. 4A, 4B show two additional example cardiac MRI images with artifacts which have been highlighted with black arrows. Once again, the artifacts can occur if the longitudinal magnetization during data readout varies from one readout to the next. It is in a transient state rather than a steady state during readout. The artifacts are known as ghosts or ghosting artifacts because the region containing the long-T1 species is visible as faint reproduction at various erroneous locations (see FIG. 4A. black arrows) throughout the image in addition to the original correct location (see FIG. 4A, white arrows). Single discrete ghosts as well as multiple ghosts can arise, see FIG. 4A. In some cases, there is no distinct ghost but signal from the long-T1 species is “smeared” across the field of view leading to an overall “unclean” impression of the image as visible in FIG. 4B.
Such ghosting image artifacts can hamper clinical image evaluation and sometimes prevent a clinical diagnosis based on the acquired MR images. For example, a ghost from pleural effusion may be placed on top of the structure of interest such as a long axis view of the heart so that a diagnosis is not possible. Even worse, they can also lead to a wrong diagnosis in for example delayed enhancement (myocardial viability) images. Smaller bright ghosts from the spinal fluid superimposed onto the myocardium may be misinterpreted as infarcts. This would lead to a false positive diagnosis and possibly inappropriate patient treatment.
Past approaches to eliminate such ghosting artifacts include use of dummy periods and the use of saturation slabs. Some current MRI pulse sequences that use an inversion recovery pulse to create image contrast play a “dummy period” at the beginning of the scan. That means, the inversion pulse and the RF and gradient pulses usually played to acquire data are played in this leading period, but no data is recorded (the recoding event is turned off, dummy data acquisition). This approach results in prolonged scan time. Furthermore, this approach generally only weakens the intensity of the ghosting artifact but does not remove it completely. The reason is that a single dummy period is not enough to drive the long-T1 species into steady state. As general rule, the purer a fluid the longer its T1, and the less effective is the product dummy period mechanism. Each dummy period increases the required breath hold time. For example, if the raw data space contains 200 lines, then in a segmented sequence these lines could be acquired in 8 segments of 25 lines each. As these sequence are usually ECG-gated and are executed with a trigger pulse of 2 meaning that each acquisition heart beat (HB) is followed by a recovery HB (to let longitudinal magnetization recover), the scan would take 2×8=16 heart beats during which the patient needs to hold her breath. Playing the dummy period (in this case dummy HB) in the beginning will add an additional two heart beats (one dummy plus one recovery HB). To fully remove the long-T1 ghosting artifact using the product mechanism one would need at least four dummy HBs leading to 4×2=8 additional heart beats. That would require a total of 24 HBs which is beyond a patient's breath hold capability. The situation would be even worse if a trigger pulse of 3 were used, because each additional dummy imaging HB would lead to two additional recovery HBs.
Another known solution is to place a saturation slab or band on top of the long-T1 species, e.g. on top of a pleural effusion or other region containing the long-T1 species. This approach works but is only possible if the long-T1 species is not part of the imaged structure. Therefore, for example, this is not possible for pericardial effusion as the band would destroy the signal of the heart as the organ of interest. Even in contexts (e.g., pleural effusion) where it is possible, this approach generally requires scanner operator skill and time to position the saturation slab and adjust its thickness. Even then, the time point when the saturation pulse is played in the pulse sequence is not optimal and artifacts may still arise. In case there is more than one region with long-T1 species, multiple saturation slabs need to be manually placed, further complicating scanner operation.
Thus, while the prior art proposed certain solutions, further improvements are possible and desirable.
Exemplary illustrative non-limiting technology herein eliminates or substantially eliminates ghosting artifacts in segmented ECG-gated and other IR-prepared sequences caused by the signal oscillations of body fluids and other matter with long T1 values by employing a long-T1 species suppression module
One exemplary illustrative non-limiting implementation herein provides a method of suppressing artifacts arising from tissue, fluids, or other long-T1 species when acquiring magnetic resonance data with a segmented pulse sequence that assumes that magnetization is at steady state, said method including suppressing artifacts by producing an artifact suppression module (ASM) before the segmented sequence, the artifact suppression module comprising at least one selective, non-selective, or volume-selective suppression pulse and an associated time delay.
The suppression pulse can for example comprise a non-selective saturation recovery (SR) pulse, an inversion recovery (IR) pulse, a partial inversion recovery pulse, or combination of suppression and inversion pulses with associated specified time delays therebetween
The segmented pulse sequence can comprise a segmented inversion recovery sequence. The segmented pulse sequence can use a gradient echo (GRE) readout, a gradient- or RF spoiled gradient echo readout, a steady state free precession (SSFP) readout, or a Turbo-spin echo (TSE) readout.
Data acquisition can comprise acquiring segmented and/or a series of singleshot images and/or a data and a reference data set comprising phase sensitive inversion recovery (PSIR).
Readout may use a Cartesian, radial, elliptical, echo planar 2D readout or a 3D readout.
A further exemplary illustrative non-limiting implementation may provide a method of suppressing artifacts arising from tissue, fluids, or other matter with long T1 value when acquiring magnetic resonance data with a pulse sequence that assumes incorrectly that magnetization is at steady state comprising: applying at least one artifact suppression pulse; waiting a delay time before or after said artifact suppression pulse to permit long T1 matter to achieve steady state magnetization to a segmented pulse sequence to follow; and then applying said segmented pulse sequence to obtain substantially steady state magnetization at readout time.
The at least one artifact suppression pulse may comprise a selective, non-selective, or volume-selective inversion recovery pulse, and said suppression pulse in combination with said time delay together comprise an artifact suppression module (ASM) that is played immediately before the segmented periodic sequence.
The at least one artifact suppression pulse may comprise a non-selective saturation recovery (SR) pulse; a selective, non-selective, or volume-selective partial inversion recovery pulse, and said inversion pulse in combination with a time delay together comprise an artifact suppression module (ASM) that is played immediately before the segmented periodic sequence; or a segmented inversion recovery sequence.
The method may further include acquiring a series of single-shot images; acquiring a data and a reference data set such as but not limited to phase sensitive inversion recovery (PSIR); and/or using a gradient echo (GRE) readout, a gradient- or RF-spoiled gradient echo readout; a steady state free precession (SSFP) readout; and/or a Turbo-spin echo (TSE) readout.
An exemplary illustrative non-limiting magnetic resonance system for imaging tissue, fluids, or long T1 matter while suppressing artifacts arising from non-steady state magnetization of said long-T1 matter during data acquisition may comprise a magnet that exposes said tissue, fluids, or other matter to a magnetic field; a radio frequency transceiver that transmits radio frequency pulses into said tissue, fluids, or other matter and receives corresponding nuclear magnetic responses from said tissue, fluids, or other matter; and a pulse sequence that controls said radio frequency transmitter to (a) apply a pre-pulse and (b) a waited a delay time before beginning a segmented pulse sequence to permit long T1 matter to achieve steady state magnetization to a segmented pulse sequence to follow; before (c) applying said segmented pulse sequence to acquire an image at substantially steady state magnetization of said long-T1 matter.
The pre-pulse may comprises an inversion recovery pulse; a partial inversion recovery pulse; a non-selective saturation recovery (SR) pulse.
A further exemplary illustrative non-limiting NMR imaging method may comprise applying a pre-pulse pulse to a subject to be imaged; waiting a delay time before or after the pre-pulse that is timed to match steady state magnetization of a portion of the subject to a segmented pulse sequence to follow; then applying said segmented pulse sequence to achieve substantially steady state magnetization at readout time; then performing an NMR readout operation; and generating an image of said subject based on said readout operation.
An example non-limiting method of suppressing artifacts arising from tissue, fluids, or other matter with long T1 value when acquiring magnetic resonance data with a pulse sequence that assumes incorrectly that magnetization is at steady state, comprises: applying a suppression pulse; waiting a delay time to permit long T1 matter to achieve steady state magnetization to a segmented pulse sequence to follow; and then applying said segmented pulse sequence to obtain substantially steady state magnetization at readout time.
Other exemplary illustrative non-limiting implementations drive the long-T1 species to steady state within one extra leading imaging period, e.g. one leading extra heart beat. Artifacts are completely removed. This is achieved by a timed saturation recovery played in the extra leading imaging period at, slightly before, or slightly after the time (relative to the beginning of the imaging period) where the inversion is played during the following imaging periods. The improvement in image quality achieved is substantial, yet the implementation is straightforward.
In some exemplary illustrative non-limiting implementations, the time delay dp is selected to be equal to di, where di is the time delay between the beginning of each acquisition period and the inversion recovery (IR) pulse. Delay dp can be also be chosen to be shorter than di, but generally should be larger positive. The exact value of di is a function of the pulse sequence parameters and the imaging period (e.g. the patient's RR interval). A precise calculation of dp can be done to optimize artifact suppression performance but is not necessary due to the robustness of the technique.
Thus, to suppress signal from long-T1 species, a non-selective saturation recovery (SR) or IR prepulse can be played. An IR or SR pulse and a time delay as a “suppression module” allows maximal recovery of normal myocardium. Long T1-species are suppressed while image SNR remains unaffected. One aspect removes ghosting artifacts in magnetic resonance images that stem from species (fluids, tissue or other matter) with long longitudinal relaxation time T1 (as found e.g. in effusions) by immediately driving the long-T1 species to steady state within one extra leading imaging period. This is achieved by playing a saturation recovery radio frequency (RF) pulse in the extra leading imaging period at or slightly before or after the time where the inversion is played during the following imaging periods. These times are relative to the beginning of each imaging period which can for example be the R-wave of the patient's electrocardiogram (ECG).
The exemplary illustrative non-limiting implementation is extremely useful and very versatile. The artifact suppression module can be played out at the beginning of many types of MRI and NMR pulse sequences. It suppresses artifacts in various regions of the human body and in any pulse sequence that acquires data in a segmented and periodic (including but not limited to gated) fashion. There is no disadvantage to the patient other than the slightly increased scan duration due to the duration of the module. Even this is not a true disadvantage as a breath hold is not required while the module is playing.
Interestingly, the exemplary illustrative non-limiting implementation often cleans up MR images even in cases where no distinct ghosting artifacts were visible without the exemplary illustrative non-limiting implementation or where poor image quality had been attributed to factors other than long-T1 species.
The exemplary illustrative non-limiting technique can work for any region of the human body and in any pulse sequence that acquires data in a segmented and periodic (e.g., including but not limited to ECG-gated) fashion. Exemplary illustrative non-limiting implementations can be used with any sort of equipment regardless of vendor, and can be realized by a small software change alone.
Other exemplary illustrative non-limiting features and advantages include:                Allows reliable artifact suppression without any user input. It uses a non-selective suppression pulse so that artifacts from all regions containing long-T1 species are simultaneously prevented.        Suppresses the artifact completely and does not just weaken it.        The increase in breath hold time is negligible and considerably shorter than previous approaches.        Works for all body regions (cardiac MR, head/neuro MR, orthopedic MR, etc.).        No reduction of signal occurs, thus there is no signal to noise (SNR) penalty.        Not limited to any specific type of data readout. SSFP (TrueFisp), gradient echo (Flash, Fast Low Angle SHot), and TSE (turbo-spin echo) readout will all show the same artifact provided that they are preceded by an inversion recovery pulse and that images are acquired in a periodic and segmented scheme.        Extremely useful and very versatile.        The artifact suppression module can be played out at the beginning of many types of MRI and NMR pulse sequences. It suppresses artifacts in various regions of the human body and in any pulse sequence that acquires data in a segmented and periodic (such as gated) fashion.        There is no disadvantage to the patient.        Often cleans up the MR image even in cases where no distinct ghosting artifacts were visible without the invention or where poor image quality had been attributed to factors other than long-T1 species.        Suppresses artifacts originating from fluids, tissue or other matter with long-T1 values referred to as “long-T1 species” such as pericardial effusion, pleural effusion, cerebro-spinal fluid, saline breast implants, and stomach fluid.        Techniques work for any region of the human body and in any pulse sequence that acquires data in a segmented and periodic (such as ECG-gated) fashion.        Vendor-independent and can be realized by a small software change alone.        